What does sleep do?

Sleep may be the most important everyday phenomenon that we understand the least well. It’s like the oceans; it’s everywhere, but we’ve only explored the surfaces. But scientists are still working to establish better knowledge of what sleep does, how it works, and why every animal does it (and needs to do it). National Geographic has a very pretty and pretty thorough summary of the latest theories of sleep scientists on what we do when we fall asleep.

The waking brain is optimized for collecting external stimuli, the sleeping brain for consolidating the information that’s been collected. At night, that is, we switch from recording to editing, a change that can be measured on the molecular scale. We’re not just rotely filing our thoughts—the sleeping brain actively curates which memories to keep and which to toss.

It doesn’t necessarily choose wisely. Sleep reinforces our memory so powerfully—not just in stage 2, where we spend about half our sleeping time, but throughout the looping voyage of the night—that it might be best, for example, if exhausted soldiers returning from harrowing missions did not go directly to bed. To forestall post-traumatic stress disorder, the soldiers should remain awake for six to eight hours, according to neuroscientist Gina Poe at the University of California, Los Angeles. Research by her and others suggests that sleeping soon after a major event, before some of the ordeal is mentally resolved, is more likely to turn the experience into long-term memories.

It’s basically a maintenance cycle. At deeper levels of sleep, we’re literally cleaning away waste products of waking life in the brain.

Good sleep likely also reduces one’s risk of developing dementia. A study done in mice by Maiken Nedergaard at the University of Rochester, in New York, suggests that while we’re awake, our neurons are packed tightly together, but when we’re asleep, some brain cells deflate by 60 percent, widening the spaces between them. These intercellular spaces are dumping grounds for the cells’ metabolic waste—notably a substance called beta-amyloid, which disrupts communication between neurons and is closely linked to Alzheimer’s. Only during sleep can spinal fluid slosh like detergent through these broader hallways of our brain, washing beta-amyloid away.

Dreams, too, reflect this heightened activity of the brain, but it’s unclear whether dreams themselves are a kind of harmless aftereffect or whether they perform a key function at reinforcing memory.

Lately, I’ve been fascinated by my own dreams, not least because reality has so often been disappointing. One recurring theme has been universities: libraries, offices, classrooms, campuses, over and over again. This isn’t terribly surprising: I spent a shade less than half my life, and most of my adult life, either going to school or working at one. But it’s difficult to make sense of. Do I miss the security/adventure of those times? Am I consolidating those memories to make room for new ones? Is an oracular entity trying to convince me to go back to school? I’m not sure. But understanding that SOMEthing is going on is a kind of balm that’s useful to me in ways I can’t totally articulate.